Inherited and environmental
Long QT syndrome
A QTc beyond 480 ms — the substrate for torsades, whether inherited or drug-induced.
60 bpmSinus, QTc 520 ms
What you see
- QTc over 450 ms in men or 460 ms in women; over 500 ms is high risk.
- Broad, late or notched T waves.
- T wave alternans, when present, signals imminent arrhythmia.
- A useful bedside check at normal rates: the QT should be less than half the RR interval.
Why it looks like that
Repolarising potassium current is reduced — by a channel mutation in congenital forms, or by drug blockade of the hERG channel in acquired ones. Repolarisation is prolonged and non-uniform, allowing early afterdepolarisations to trigger torsades.
What to do
- Stop every QT-prolonging drug; correct potassium, magnesium and calcium.
- Beta-blockade (propranolol or nadolol) for congenital long QT; avoid competitive sport in LQT1.
- ICD for those who have had cardiac arrest or syncope despite beta blockade.
- Screen and counsel first-degree relatives.
The trap
Use Bazett's correction with care: it over-corrects at fast rates and under-corrects at slow ones, so a tachycardic patient can be labelled with a long QT they do not have. Also remember the trigger pattern — LQT1 with exercise and swimming, LQT2 with startle and emotion, LQT3 during sleep.
QTcBazetthERGbeta-blocker